The invention relates to an electric motor with a frequency converter for speed control, a casing of the frequency converter being provided with a cooler or dissipator.
The term converter is understood to mean a transformer, which transforms the mains voltage with constant frequency and voltage into a three-phase voltage system with a variable frequency and variable voltage. This three-phase voltage system is supplied to the three-phase motor (asynchronous machine) to be driven, so as in this way to predetermine the electric motor speed by a corresponding choice of the output frequency of the converter.
Unlike in the conventional arrangement, in which the converter is arranged as a separate unit in a switch cabinet and supplies the electric motor by means of a motor lead with a length of up to 100 m, electric motors are already known in which the electric motor-controlling signal and power electronics is integrated as a mechanical unit into the electric motor. This economizes on the previously necessary switch cabinet space requirement and also reduces EMC problems, which were caused in the conventional arrangement by radiation as a result of the long motor leads with a length of up to 100 m. For integrating the frequency converter into the electric motor the electronics thereof are installed in one of the terminal boxes of the electric motor. The connecting elements such as terminals and/or plugs for the supply and signal lines are also located within the terminal box. Particularly with regards to the manufacturing technology and the associated manufacturing costs, such an arrangement in which the frequency converter and electric motor are constructionally interconnected offers numerous advantages. However, problems arise with the cooling of the frequency converter, because in particular the power output stage of the power electronics produces high waste heat levels. In order to dissipate the frequency converter waste heat, on the B side of the electric motor (the B side is the side remote from the driven motor shaft end) a separate fan cowl with fan must be fitted, said fan being driven by the electric motor or by an additional, separate fan motor.
In place of integrating the frequency converter into the terminal box, the frequency converter casing can also be fitted to the electric motor on the B side. Here again for dissipating the frequency converter waste heat a fan must be fitted to the electric motor on the B side and is here again driven by the electric motor or by a separate fan motor. In this variant only the connecting elements such as terminals and/or plugs for the supply and signal lines are housed in one or more terminal boxes, so that they maintain their standard size. However, through the incorporation of the electronics on the B side end of the electric motor, the overall length of the electric motor is now increased. A further increase in the overall length results from the necessary special fan cowl. If no fan is provided, the electronics must be designed for high temperatures and special high temperature resistant, electronic components are used.
As a result of the described special parts when integrating the converter into the electric motor, it is not possible to use a simple standard motor. For the fitting of the frequency converter and special fan cowl and optionally fan motor special structures are required on the electric motor. Apart from the space requirement caused by the additional fan, additional disturbing fan noise arises and the energy costs for the fan as an active cooling element is extremely high. The separate motor required for the fan is also an expensive component.